This invention relates to an operational amplifier.
As is known, operational amplifiers find large-scale application in various branches of electronics thanks to their inherent characteristics, versatility and the possibility of providing characteristics of the output signal which differ according to their configuration and connection with other components.
A typical (inverting) configuration is shown in FIG. 1 where the operational amplifier 1 has at its non-inverting input a voltage V.sub.L, its output, at the voltage V.sub.o, is fed back to the inverting input of the amplifier 1 through the resistor R.sub.2, and the input signal V.sub.IN is applied to the same inverting input through the resistor R.sub.1. Such an operational amplifier has an approximately linear behavior within a voltage range limited by inherent factors to the operational amplifier itself.
However, it is occasionally desirable to limit the output range between set values to be selected at will.
To attain a limitation of the output range, prior circuits often adopt the solution of arranging, in parallel with the feedback resistor, a pair of antiparallel-connected zener diodes.
Consequently, as long as the output voltage is lower than the zener breakdown voltage, the zeners do not intervene and the operational amplifier behaves normally.
When, on the contrary, the output voltage tends to exceed the breakdown voltage of either zener, this is subjected to breakdown, thereby the feedback network will have a low impedance which reduces the gain and causes an output voltage limitation down to the zener breakdown value.
However, that approach has some disadvantages.
In particular, it is not possible to exactly set the limit voltage or voltages of the output range for two different reasons.
First, when utilizing zeners, the limitation voltages can only have discrete values and cannot be varied continuously according to contingent needs.
Furthermore, the limitation voltages cannot be set accurately due to the spread of the breakdown voltages in the diode manufacture and are not constant, due to their drift with temperature.
While are known circuit solutions tending to reduce the drift of the zener breakdown voltage with temperature, such solutions cannot solvet the first problem, do not allow precise set of the voltage limits and on the other hand, they introduce considerable circuit complexity into the amplifier, with evident disadvantages.